RU2660478C2 - Chromium-chromium oxide coatings applied to steel substrates for packaging applications and method for producing said coatings - Google Patents

Abstract

FIELD: electrolytic methods.

SUBSTANCE: invention relates to electroplating and can be used for packaging applications. Method comprises electrolytically depositing on said substrate said chromium metal - chromium oxide coating in a single process step from an electrolyte comprising basic chromium sulphate, a chelating agent and a conductivity increasing salt, wherein the cation in the chelating agent and the conductivity increasing salt is an alkali metal cation, the chelating agent contains an anion of formic acid, and to ensure chromium deposition, the threshold density of the cathode current is exceeded to reach a predetermined pH at the substrate surface which is higher than the pH in the electrolyte volume. Resulting substrate is further provided with a thermoplastic monolayer or multilayer polymer coating comprising thermoplastics such as polyesters or polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon polymers, polycarbonates, styrene resins, ABS plastics, chlorinated polyethers, ionomers, urethane resins and functionalised polymers, and / or copolymers thereof and / or mixtures thereof.

EFFECT: technical result: the creation of a substrate, not associated with the use of Cr (VI) during its production.

20 cl, 7 tbl, 5 dwg

Description

[0001] This invention relates to coatings of chromium-chromium oxide (Cr-CrOx) deposited on steel substrates for packaging applications, and to a method for producing said coatings.

[0002] Hard-rolling products include tinplate, electrolytically chrome-plated steel (ECCS, also referred to as tin-free tin or “TFS” for short) and black tin, i.e. uncoated steel. Packaging steels are usually supplied in the form of tinplate or ECCS, which can be coated with an organic coating. In the case of tin plate is an organic coating is usually a paint, while in the case of ECCS used more polymeric coatings, such as PET or PP, as in the case Protact ^®.

[0003] Tinplate is characterized by excellent corrosion resistance and weldability. Tinplate is usually supplied in a coating weight range between 1.0 and 11.2 g / m ² , which is typically applied by electroplating. Currently, most of the tinplate is subjected to subsequent treatment with fluids containing hexavalent chromium, Cr (VI), by dipping or electrolytic application process. The purpose of this post-treatment is to passivate the tin surface to stop or reduce the growth of tin oxides, as too thick oxide layers can ultimately lead to problems associated with the adhesion of organic coatings such as varnishes. It is important that such passivation treatment should not only inhibit or eliminate the growth of tin oxide, but should also be able to maintain or improve the adhesion level of the organic coating. The passivated outer surface of tinplate is extremely thin (less than 1 micron thick) and consists of a mixture of tin and chromium oxides.

[0004] ECCS consists of a product - black sheet, on which a layer of metallic chromium is coated, coated on top with a film of chromium oxide, both of which are deposited by electrolytic deposition. ECCS exhibits excellent adhesion to organic coatings, maintaining the integrity of coatings at temperatures exceeding the melting point of tin (232 ° C). In these cases, tin-coated (tinned) material must not be used. This is important for the production of polymer-coated packaging steel, since during the application of thermoplastic coatings, the steel substrate can be heated to temperatures exceeding 232 ° C, and the actual maximum temperature used depends on the type of thermoplastic coating applied. This heating cycle is necessary to provide the initial thermal bonding / bonding of thermoplastics with the substrate (preliminary heat treatment), and it is often followed by subsequent heat treatment to modify the properties of the polymer. It is believed that the chromium oxide layer is responsible for the excellent adhesive properties of thermoplastic coatings, such as polypropylene (PP) or polyethylene terephthalate (PET), to ECCS. ECCS can also be supplied with both Cr and CrOx coatings with coating weights within a certain range, typically between 20-110 and 2-20 mg / m ^2, respectively. ECCS can be delivered with the same coating specifications for both sides of the steel strip or with different coating weights on different sides, in the latter case we speak of a strip with a differential coating. ECCS production currently includes the use of chromium-based solutions in its hexavalent state, also known as hexavalent chromium or Cr (VI).

[0005] Hexavalent chromium is currently regarded as a hazardous substance that is potentially harmful to the environment and represents a risk factor for worker safety. Thus, there is an incentive to develop alternative metal coatings that would be able to replace traditional tinplate and ECCS and eliminate the need for hexavalent chromium production.

[0006] An object of the invention is to provide an alternative to the use of hexavalent chromium for passivation of tinplate.

[0007] An object of the invention is to provide an alternative to traditional tinplate to improve product properties, for example with regard to corrosion performance and resistance to staining with sulfur.

[0008] Another objective of the invention is to provide an alternative substrate for tinplate and ECCS, providing excellent dry adhesion to organic coatings in combination with corrosion protection, which is not associated with the use during the production of hexavalent chromium.

[0009] One or more of these problems can be solved by obtaining a packaging steel substrate containing:

1. Subjected to recrystallization annealing, a substrate for single or double rolling of packaging steel (ie, black sheet), or

2. Cold-rolled and re-annealed black sheet,

moreover, one or both sides of the substrate is coated with a coating layer of metal chromium chromium oxide (Cr-CrOx) obtained in a single technological stage of deposition using the process of electrodeposition of trivalent chromium.

[0010] The packaging steel substrate is preferably made in the form of a strip.

[0011] Three types of chromium electrodeposition methods are commonly used around the world for the production of ECCS. These three methods are the "one-stage vertical method" (V-1), the "two-stage vertical method" (V-2), and the "one-stage horizontal method with a high current density" (CAP) with Cr (VI) -based electrolytes. ECCS specifications are standardized in accordance with Euronorm EN 10202: 2001. In a two-step vertical process, a sulfuric acid-free electrolyte based on Cr (VI) is used to deposit a layer of chromium oxide in a second step. Sulfuric acid is necessary for good effectiveness in the deposition of metallic chromium and, therefore, it is always used in the deposition of metallic chromium in these methods. The “one-stage vertical” and “one-stage horizontal high current density (CAP)” methods always lead to the presence of sulfate in the oxide layer, since metallic chromium and chromium oxide are produced simultaneously in the same electrolyte (Boelen, dissertation TU Delft 2009, p. 8-9, ISBN 978-90-805661-5-6). In all cases, ECCS consists of a layer of chromium oxide on top of metallic chromium.

[0012] In the method according to the invention, a coating layer comprising metal chromium and chromium oxide is precipitated, and a metal chromium layer is not precipitated first, followed by a chromium oxide layer as a conversion layer. The Cr-CrOx layer should consist of a mixture of Cr oxide and metallic Cr, and Cr oxide should not be present as a separate layer on the outer surface, but should be mixed throughout the Cr-CrOx layer. Of course, there can be more than one such single deposition step, one after another, if, for example, a thicker coating layer containing a layer of metallic chromium and chromium oxide is to be deposited. The phrase “single electrodeposition stage”, therefore, does not always mean that only one of these single electrodeposition stages is used.

[0013] The packaging steel substrate is usually made in the form of a strip of low carbon (LC), extra low carbon (ELC) or ultra low carbon (ULC) steel with a carbon content in weight percentages between 0.05 and 0.15 (LC), between 0.02 and 0.05 (ELC) or less than 0.02 (ULC), respectively. Alloying elements such as manganese, aluminum, nitrogen, but sometimes also elements such as boron, are added to improve mechanical properties (see also, for example, EN 10 202, 10 205 and 10 239). In one embodiment of the present invention, the substrate consists of non-embedding elements of low-carbon, extra-low-carbon or ultra-low-carbon steel, such as titanium stabilized, niobium stabilized or titanium-niobium stabilized steel without embedded elements.

[0014] It has been found that a metallic chromium-chromium oxide (Cr-CrOx) coating obtained by electrodeposition from a trivalent chromium electrolyte provides excellent adhesion to organic coatings. In this aspect, a metal chromium-chromium oxide (Cr-CrOx) coating obtained by electrodeposition from an electrolyte based on trivalent chromium has adhesive properties quite similar to those of traditional ECCS made by the hexavalent chromium deposition process. As the thickness of the Cr-CrOx coating layer increases, the porosity of the coating decreases, and its anticorrosion properties improve.

[0015] The Cr-CrOx coating can be applied to a conventional, non-passivated, electrolytic and, optionally fused, tinplate (ETP, electrolytically tinplate). The Cr-CrOx layer suppresses the growth of tin oxides, i.e. performs a passivating function. As the thickness of Cr-CrOx increased, it was unexpectedly found that wet adhesion characteristics, i.e. the adhesion of the organic coating after sterilization is better than in the case of traditional tinplate passivated by hexavalent chromium. In addition, resistance to so-called sulfur staining, i.e. brown tinplate darkening due to contact with sulfur-containing fillers can be completely suppressed by applying a sufficiently thick Cr-CrOx coating. The material in accordance with the invention is thus well suited to replace tinplate passivated with hexavalent chromium, not necessarily exceeding the limit specifications of standard tinplate. From a technological point of view, the fact that the Cr-CrOx coating layer is applied in one single technological stage means that the two technological stages are combined, which is useful in terms of process efficiency and environmental impact.

[0016] Alternatively, the Cr-CrOx coating can also be applied directly to the packaging steel substrate — black sheet, without first applying a tin coating, i.e. directly onto a bare steel surface. According to Merriam Webster, tinplate is defined as sheet steel that has not yet been tinned by tin coating or which is used without coating when tin protection is not required. It was found that the level of dry adhesion of this material to organic coatings both in the case of thermosetting varnishes and in the case of thermoplastic coatings can approach the value usually associated with the use of ECCS. The material according to the invention can be used as a direct replacement for ECCS for applications that require moderate corrosion resistance.

[0017] A great advantage both in terms of environmental impact and in terms of health and safety is that the present invention eliminates the use of hexavalent chromium, while it is possible to maintain the performance properties of products typically found in ECCS and tinplate.

[0018] In one embodiment, the Cr-CrOx coating layer applied to the non-passivated tinplate contains at least 20 mg Cr / m ² to create a tin oxide passivation effect. This thickness is suitable for many purposes.

[0019] In an embodiment, the Cr-CrOx coating layer applied to the non-passivated tinplate contains at least 40 mg Cr / m ² , preferably at least 60 Cr / m ² , to provide a tin oxide passivation effect and to prevent or eliminate staining with sulfur. It was found that to prevent or eliminate staining, the gray layer with 20 mg Cr / m ^{2 is} too thin. Starting with thicknesses of about 40 mg Cr / m ^2, sulfur staining is already markedly reduced, while with a layer thickness of at least about 60 mg Cr / m ^2, sulfur staining is virtually eliminated.

[0020] A suitable maximum thickness was found to be 140 mg Cr / m ² . Preferably, the Cr-CrOx coating layer applied to the non-passivated tinplate contains at least 20 and up to 140 mg Cr / m ² , more preferably at least 40 and / or not more than 90 mg Cr / m ² , and most preferably at least 60 and / or not more than 80 mg Cr / m ² .

[0021] The purpose of these embodiments is to replace tinplate passivated with hexavalent chromium. The main advantage besides the exclusion of hexavalent chromium from production is the potential possibility of creating a product with excellent resistance to gray staining and improved corrosion resistance.

[0022] It was found that the color of the material changes as the thickness of the Cr-CrOx layer increases, and as the coating thickness increases, the product becomes darker (ie, the L value decreases). Since the optical properties of packaging steels are very important for creating an aesthetically attractive appearance of metal containers, such as aerosol containers, this can be considered as a disadvantage of the invention in relation to some special applications. However, one way to circumvent these problems would be to apply a differential coating, for example, to use a Cr-CrOx coating with a low weight on one side of the material while simultaneously applying a thicker Cr-CrOx coating on the other side. A thicker Cr-CrOx surface should be used for the inside of the container, taking advantage of the improved corrosion resistance properties. In this case, the surface with a thinner Cr-CrOx coating is located on the outside of the container, for which the requirements for corrosion resistance are usually less stringent, which ensures optimal optical properties.

[0023] In an embodiment, the Cr-CrOx coating layer deposited on the black sheet is at least 20 mg Cr / m ² , which gives a material that is close in functionality to ECCS (for example, it has excellent adhesion to organic coatings in combination with moderate corrosion resistance). Preferably, the Cr-CrOx coating layer deposited on the black sheet is at least 40, and more preferably at least 60 mg Cr / m ² . A suitable maximum thickness was 140 mg Cr / m ² . Preferably, the Cr-CrOx coating layer deposited on the black sheet contains at least 20 and up to 140 mg Cr / m ² , more preferably at least 40 mg Cr / m ² , and most preferably at least 60 mg Cr / m ² . In one embodiment, a suitable maximum is 110 mg Cr / m ² .

[0024] Coated Cr-CrOx black sheet is intended to replace ECCS. The main advantage besides exclusion from hexavalent chromium production is the potential possibility of creating a product for those applications in which tinplate does not require excellent corrosion resistance. From a technological point of view, the fact that the Cr-CrOx coating layer is applied in one single technological stage means that the two technological stages are combined, which is useful in terms of process efficiency and environmental impact.

[0025] The Cr-CrOx coating can also be applied to cold rolled and re-annealed black sheet or to cold-rolled and re-annealed electrolytic, and optionally fused, white sheet. These substrates are reductive annealed substrates rather than recrystallized white (ETP) or black single sheet tin or black double sheet. It was not found that the difference in the microstructure of the substrate significantly affects the Cr-CrOx coating.

[0026] It was found that the material according to the invention can be used in combination with thermoplastic coatings, but also in applications where ECCS is traditionally used in combination with varnishes (ie, for heat-resistant dishes, such as baking molds, or products with moderate requirements for corrosion resistance), or as a substitute for traditional tinplate for applications with moderate requirements for corrosion resistance.

[0027] In one embodiment, the coated substrate is further provided with an organic coating consisting of either a thermosetting organic coating, or a thermoplastic single-layer polymer coating, or a thermoplastic multilayer polymer coating. The Cr-CrOx layer provides excellent adhesion to the organic coating, similar to that achieved using traditional ECCS.

[0028] In a preferred embodiment, the thermoplastic polymer coating is a polymer coating system comprising one or more layers using thermoplastics such as polyesters or polyolefins, but it may also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon polymers, polycarbonates, styrene resins, ABS plastics, chlorinated polyethers, ionomers, urethane resins and functionalized polymers. For clarity:

[0029] A polyester is a polymer consisting of dicarboxylic acid and glycol. Examples of suitable dicarboxylic acids include terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid and cyclohexanedicarboxylic acid. Examples of suitable glycols include ethylene glycol, propanediol, butanediol, hexanediol, cyclohexanediol, cyclohexanedimethanol, neopentyl glycol, etc. More than two types of dicarboxylic acids or glycols can be used simultaneously.

[0030] Polyolefins include, for example, polymers or copolymers of ethylene, propylene, 1-butene, 1-pentene, 1-hexene or 1-octene.

[0031] Acrylic resins include, for example, polymers or copolymers of acrylic acid, methacrylic acid, acrylic ester, methacrylic ester or acrylamide.

[0032] Polyamide resins include, for example, the so-called Nylon 6, Nylon 66, Nylon 46, Nylon 610, and Nylon 11.

[0033] Polyvinyl chloride includes homopolymers and copolymers, for example, with ethylene and vinyl acetate.

[0034] Fluorocarbon polymers include, for example, tetrafluorinated polyethylene, trifluorinated monochlorinated polyethylene, hexafluorinated ethylene-propylene resin, polyvinyl fluoride and polyvinylidene fluoride.

[0035] Functionalized polymers, for example obtained by grafting maleic anhydride, include, for example, modified polyethylenes, modified polypropylenes, modified ethylene-acrylate copolymers, and modified ethylene vinyl acetate.

[0036] Mixtures of two or more resins may be used. In addition, the resin can be mixed with an antioxidant, a heat stabilizer, a UV absorber, a plasticizer, a pigment, a nucleating agent, an antistatic agent, an anti-adhesive, an anti-stick additive, etc. It has been shown that the use of such systems of thermoplastic polymer coatings provides excellent performance in the manufacture of cans and the use of cans, for example, in relation to the shelf life.

[0037] In accordance with a second aspect, the invention is embodied in a method for producing a coated steel substrate for packaging applications, comprising electrodepositing a metal chromium chromium oxide coating on a substrate, wherein the electrolytic deposition of said metal chromium chromium oxide coating on said substrate occurs in a single electrodeposition step from an electrolyte containing a trivalent chromium compound, an optional chelating agent, an optional conductivity enhancing salt, an optional depot yarizator, optional surfactant, and to which may be added acid or base to adjust the pH.

[0038] In an embodiment, the electrodeposition of a Cr-CrOx coating is carried out using an electrolyte in which the chelating agent contains formic acid anion, the conductivity increasing salt contains an alkali metal cation, and the depolarizer contains a bromide-containing salt.

[0039] In an embodiment, the cation in the chelating agent that increases the salt conductivity and the depolarizer is potassium. The advantage of using potassium is that its presence in the electrolyte significantly increases the conductivity of the solution, more than the cation of any other alkali metal, which provides the maximum contribution to the decrease in the cell voltage required for the course of the electrodeposition process.

[0040] In one embodiment, the composition of the electrolyte used to precipitate Cr-CrOx was as follows: 120 g / L basic chromium sulfate, 250 g / L potassium chloride, 15 g / L potassium bromide and 51.2 g / L potassium formate. The pH was adjusted to between 2.3 and 2.8, measured at 25 ° C, by the addition of sulfuric acid.

[0041] According to the invention, the chromium-containing coating is preferably deposited from a trivalent chromium-based electrolyte at a bath temperature between 40 and 70 ° C, preferably at least 45 ° C and / or not more than 60 ° C.

[0042] It has been unexpectedly discovered that it is possible to electrodeposit the metallic chromium-chromium oxide coating layer from this electrolyte in one process step. From the prior art it follows that the addition of a buffering agent to the electrolyte, such as, for example, boric acid, is absolutely necessary to enable electrodeposition of metallic chromium. In addition, it was reported that it is impossible to precipitate metallic chromium and chromium oxide from the same electrolyte because of this buffering effect (a buffering agent is required for the electrodeposition of metallic chromium, but excludes the formation of chromium oxides, and vice versa). However, it was found that such a buffer addition was not required for the deposition of chromium metal, provided that a sufficiently high cathode current density was used.

[0043] The depth profiles were measured by X-ray photoelectron spectroscopy (XPS), and the measured peaks corresponded to Fe2p, Cr2p, O1s, Sn3d, C1s. It was noted that the Cr layer consists of a mixture of Cr oxide and metallic Cr and that Cr oxide is not present as a separate layer on the outer surface, but is mixed throughout the layer. This is also indicated by the peak O present throughout the Cr layer. In all cases, the Cr-CrOx layer has a shiny metallic appearance.

[0044] It is believed that for the deposition of metallic chromium, it is necessary to exceed a certain threshold value of the current density, which is directly related to the pH at the strip surface, reaching certain values as a result of the release of gaseous hydrogen and the establishment of equilibrium of various (chelated) polyhydroxide chromium complexes. It was found that after passing through this threshold value of the current density, the electrodeposition of the coating layer metal chromium-chromium oxide increases almost linearly with increasing current density, as is observed in the case of conventional electrodeposition of metals, in accordance with the Faraday law. The actual value of the threshold current density is apparently directly related to the conditions of mass transfer on the strip surface: it was noted that this threshold value increases with an increase in the mass transfer rates. This phenomenon can be explained by changes in the pH values on the strip surface: with increasing mass transfer rates, the influx of hydroxonium ions to the surface increases, necessitating an increase in the cathode current density to maintain a given pH level (clearly higher than the pH in volume) on the strip surface under steady conditions process. The validity of this hypothesis is evidenced by the results obtained during experiments in which the pH of the bulk electrolyte was changed in the range of values from 2.5 to 2.8: the threshold value of the current density decreases with increasing pH.

[0045] Regarding the process of electrodeposition of Cr-CrOx coatings from trivalent chromium electrolytes, it is important to prevent / minimize the oxidation of trivalent chromium to its hexavalent state at the anode, and a suitable anode or anode material should be selected. By using a hydrogen gas diffusion anode, as described below and in co-filed EP 12193623, the formation of Cr (IV) can be prevented.

[0046] In an embodiment of the invention, the formation of Cr (IV) can be prevented by using one, more than one or a single hydrogen gas diffusion anode on which hydrogen gas is oxidized (H ₂ (g)). H ⁺ (protons) in an aqueous solution bind to one or more water molecules, for example, in the form of hydroxonium ions (H ₃ O ⁺ ). Oxidation of H ₂ (g) to H ⁺ (water) prevents undesired oxidation reactions, such as the formation of Cr (IV), which occurs at a higher anode overvoltage when using an anode on which water (H ₂ O) is oxidized to oxygen (O ₂ (g)).

[0047] The reaction H ₂ (g) → 2H ⁺ (water) + 2e ^- proceeds at an anode potential of 0.00 V (CBE). The reaction 2H ₂ O → 4H ⁺ (water) + O ₂ (g) + 4e ^- proceeds at an anodic potential of 1.23 V (SVE). When using an anode on which water is oxidized to oxygen, reactions are possible that would not be possible when using an anode on which hydrogen gas is oxidized.

[0048] One such undesirable oxidation reaction is the oxidation of Cr (III) to Cr (VI), and this oxidation reaction can be completely suppressed by using a hydrogen gas diffusion anode (GDA) on which H ₂ (g) is oxidized to H ⁺ .

[0049] In one embodiment of the method, H ₂ (g) is oxidized at the gas diffusion anode to H ⁺ (water) in a current output of at least 99%, preferably 100%. The greater the current efficiency, the less the likelihood of unwanted adverse reactions. Thus, it is preferred that the current efficiency is at least 99%, and preferably 100%. Based on thermodynamic and kinetic considerations, it can be concluded that the use of a hydrogen gas diffusion anode completely eliminates the risk of Cr (III) oxidation, since the working potential of the anode is too low for Cr (III) oxidation to occur.

[0050] Thermodynamically, under standard conditions (ie, a temperature of 25 ° C and a pressure of 1 atm), an electrode potential> 0 V is already sufficient for oxidation of H ₂ (g) to H ⁺ (water), while for oxidation of H ₂ O to O ₂ (g) requires an electrode potential> 1.23 V. Cr (III) can only be oxidized to Cr (VI) when the electrode potential> 1.35 V.

[0051] The electrode potential was measured relative to a standard hydrogen electrode. A standard hydrogen electrode (SVE) is a redox electrode that underlies the thermodynamic scale of redox potentials. Its absolute electrode potential was estimated to be 4.44 ± 0.02 V at 25 ° C, but in order to provide a basis for comparison with all other electrode reactions, the potential of a standard hydrogen electrode (E ⁰ ) is assumed to be zero at all temperatures. The potentials of any other electrodes are compared with the potential of a standard hydrogen electrode at the same temperature.

[0052] The predominant equilibrium (at zero current) potential can be calculated by the Nernst equation, substituting the corresponding temperature, pressure and activity of electroactive particles. The anode working potential (at a non-zero current) necessary to create a specific anode current is determined by the activation overvoltage (i.e., the potential difference required for the electrode reaction) and the concentration overvoltage (i.e. the potential difference required to compensate for concentration gradients electroactive particles on the electrode).

[0053] Due to the low anode overvoltage required for the oxidation of H ₂ (g) to H ⁺ (water), the working anode potential will always be much lower than the value at which Cr (III) oxidation can occur (see FIG. 4, where The dependence of the current on the anode potential relative to the SHE is shown). Firstly, this leads to less energy consumption during electrodeposition. Secondly, at an anode potential below about 1.35 V, oxidation of Cr (III) to Cr (VI) is not possible (indicated by a crossed arrow).

[0054] In one embodiment, a depolarizer is not added to the electrolyte. When using a hydrogen gas diffusion anode, the addition of a depolarizer to the electrolyte is no longer required.

[0055] The use of a hydrogen gas diffusion anode has the additional advantage that it becomes possible to use a chloride-containing electrolyte without the risk of chlorine formation. This chlorine gas is potentially harmful to the environment and workers and therefore undesirable. This means that in the case of an electrolyte with Cr (III), such an electrolyte can be partially or completely based on chlorides. The advantage of using an electrolyte based on chlorides is that the conductivity of the electrolyte is much higher than in the case of an electrolyte based only on sulfates, which leads to a lower cell voltage required for the deposition, i.e. to lower energy consumption.

[0056] The oxidation reaction of dissolved hydrogen on the active surface of the electrocatalyst is a very fast process. Since the solubility of hydrogen in a liquid electrolyte is often low, this oxidation reaction can easily become limited by the limitations of mass transfer. To overcome the limitations of mass transfer, porous electrodes have been specially developed. A hydrogen gas diffusion anode is a porous anode containing a three-phase interface between gaseous hydrogen, a liquid electrolyte and a solid electrocatalyst (e.g., platinum), which is deposited on an electrically conductive porous matrix (e.g., porous carbon or porous foam metal). The main advantage of using such a porous electrode is that it provides a very large internal surface area for a reaction enclosed in a small volume, combined with a significantly reduced diffusion path from the gas / liquid interface to the reaction centers. Due to this design, the mass transfer rate of hydrogen increases significantly, while the true local current density decreases at a given total current density on the electrode, which leads to a lower electrode potential.

[0057] The complete gas diffusion anode used in the proposed electrodeposition process typically includes the following functional components (see FIG. 5): gas supply chamber 1, current collector 2, and gas diffusion anode consisting of a hydrophobic porous gas diffusion transport layer 3 in combination with a hydrophilic reaction layer 4 (see Fig. 5). The latter is a network of micropores that are (partially) flooded with liquid electrolyte. Optionally, the reaction layer on the outside is provided with proton exchange membrane 5, for example, membrane Nafion (Nafion ^®), in order to prevent the diffusion of chemicals (such as anions or large neutral molecules) present in the volume of the liquid electrolyte inside of the gas diffusion anode, since these compounds can potentially poison the electrocatalytic centers, causing a deterioration in electrocatalytic activity.

[0058] The main function of the gas supply chamber is to uniformly supply hydrogen gas to the hydrophobic back side of the hydrogen gas diffusion anode. Two connections are required for the gas supply chamber: one to supply hydrogen gas and one to allow small amounts of hydrogen gas to be purged to prevent the accumulation of gas-phase contaminants potentially present in trace amounts in the supplied hydrogen gas. The hydrogen supply chamber often contains a channel-type structure that provides a uniform distribution of hydrogen gas over the hydrophobic back side.

[0059] A collector 2 (usually) is attached to the hydrophobic rear side 3 of the hydrogen gas diffusion anode to allow the electric current generated inside the anode to be transferred to a rectifier (not shown in FIG. 5). This collector plate should be made in such a way as to allow gaseous hydrogen to contact the rear side of the hydrogen gas diffusion anode so that it can be transported to the reactive side inside the gas diffusion anode. This is usually achieved by using an electrically conductive plate with a large number of holes, a mesh or expanded metal sheet made, for example, of titanium.

[0060] The functionality of the gas supply channels and the collector can also be combined into a single component, which in this case is pressed against the hydrophobic back side of the gas diffusion anode.

[0061] As gaseous hydrogen diffuses through the hydrophobic back side of the hydrogen gas diffusion anode, it comes into contact with an electrolyte present in the hydrophilic part of the anode, i.e. with a reaction layer (see Fig. 5, right). At the gas / liquid interface (between 3 and 4), gaseous hydrogen is dissolved in the electrolyte and transferred through diffusion to the electrocatalytically active centers of the hydrogen gas diffusion anode. Typically, platinum is used as the electrocatalyst, but other materials such as platinum-ruthenium alloys or platinum-molybdenum can also be used. At electrocatalytic centers, dissolved hydrogen is oxidized: the formed electrons are transferred through the conductive matrix of the gas diffusion anode (usually a carbon matrix) to collector 2, while hydroxonium ions (H ⁺ ) diffuse through the proton exchange membrane into the electrolyte.

[0062] In one embodiment, the coated substrate is further provided on one or both sides with an organic coating consisting of a thermosetting organic coating in a varnishing step, or with a thermoplastic single layer or thermoplastic multilayer polymer coating at the film pressing (lamination) stage or direct extrusion stage .

[0063] In one embodiment, the thermoplastic polymer coating is a polymer coating system comprising one or more layers using thermoplastics such as polyesters or polyolefins, but may also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon polymers, polycarbonates, styrene resins, ABS plastics, chlorinated polyethers, ionomers, urethane resins and functionalized polymers; and / or their copolymers; and / or mixtures thereof.

[0064] Preferably, the substrate is cleaned before electrodeposition of Cr-CrOx by immersing the substrate in a sodium carbonate solution containing from 1 to 50 g / l Na ₂ CO ₃ at a temperature of from 35 to 65 ° C, and a cathodic current density of from 0 is supplied , 5 and 2 A / dm ² for a period of time from 0.5 to 5 s.

[0065] Preferably, the sodium carbonate solution contains at least 2 and / or not more than 5 g / l Na ₂ CO ₃ .

[0066] The invention is further explained by the following non-limiting examples and drawings.

[0067] Example 1 (not part of the invention): Sheets of traditional, non-passivated, fused tinplate (steel of ordinary grade and tempering) with a tin coating weight of 2.8 g Sn / m ² on both sides were first subjected to electrolytic pre-treatment to minimize layer thickness tin oxide. This was done by immersing the sheets in a solution of sodium carbonate (3.1 g / l Na ₂ CO ₃ , temperature 50 ° C) and applying a cathode current density of 0.8 A / dm ² for 2 s. After washing with deionized water, the samples were immersed in an electrolyte with trivalent chromium, maintained at 50 ° C and consisting of: 120 g / l of basic chromium sulfate, 250 g / l of potassium chloride, 15 g / l of potassium bromide and 51.2 g / l of formate potassium. The pH of this solution was adjusted to 2.3, measured at 25 ° C, by the addition of sulfuric acid. A Cr-CrOx coating containing 21-25 mg Cr / m ² (measured by X-ray fluorescence, XRF) was deposited onto the surface by applying a cathodic current density of 10 A / dm ^{2 for} about 1 s using a platinum titanium anode as a counter electrode. The samples thus obtained possessed a metallic luster.

[0068] In the study of the passivating effect of a thin coating of Cr-CrOx on tinplate, the samples were tested for long-term storage at 40 ° C with a static level of relative humidity (RH) of 80%. Then measure the amount of tin oxide formed on the surface of tinplate during storage after 2 weeks and after 4 weeks of exposure, and compare with the amount of tin oxide present on the sample before storage tests (indicated as "0 weeks"). The thickness of the tin oxide layer was determined by coulometric method, as described by SC Britton, “Tin vs corrosion”, ITRI Publication No. 510 (1975), Chapter 4. The tin oxide layer was reduced with a controlled small cathodic current of 0 , 1% -7% solution of hydrobromic acid (HBr), which is purified from oxygen by purging with nitrogen. The development of oxide reduction is monitored by measuring the potential, and the charge that goes to full recovery (expressed in coulombs per m ² or C / m ² ) serves as a measure of the thickness of the tin oxide layer. The results for the sample of Example 1 are presented in table 1, including the characteristics of the control material, which is the same tin plate that was passivated using hexavalent chromium, i.e. the so-called passivated tinplate 311.

[0069]

Table 1 Tin oxide layer thickness (in C / m ² ) Storage at 40 ° C, 80% RH ETP-311 (control) ETP - Cr-CrOx according to Example 1 (25 mg / m ² Cr) 0 weeks 12 eleven 2 weeks 12 12 4 weeks 13 eleven

[0070] The results show that the non-passivated tinplate, treated in accordance with the present invention to obtain a light coating of Cr-CrOx, has perfect stability with respect to the growth of tin oxide and is completely comparable in its characteristics with traditional passivated white tin 311.

[0071] Example 2 (not part of the invention): Sheets of traditional, non-passivated, fused tinplate (steel of ordinary grade and tempering), with a tin coating weight of 2.8 g Sn / m ² on both sides, were first subjected to electrolytic pre-treatment to minimize tin oxide layer thickness. This was done by immersing the sheets in a solution of sodium carbonate (3.1 g / l Na ₂ CO ₃ , temperature 50 ° C) and applying a cathode current density of 0.8 A / dm ² for 2 s. After washing with deionized water, the samples were immersed in an electrolyte with trivalent chromium, maintained at 50 ° C and consisting of: 120 g / l of basic chromium sulfate, 250 g / l of potassium chloride, 15 g / l of potassium bromide and 51.2 g / l of formate potassium. The pH of this solution was adjusted to 2.3, measured at 25 ° C, by the addition of sulfuric acid. A Cr-CrOx coating containing 65-75 mg Cr / m ² (measured by XRF) was deposited onto the surface by applying a cathode current density of 15 A / dm ^{2 for} approximately 1 second using a platinum titanium anode as a counter electrode. All samples thus obtained possessed a metallic luster. In FIG. Figures 1 and 2 show a typical SEM image that depicts the deposition of very finely dispersed grains of metallic chromium-chromium oxide on a tin surface.

[0072] The sheets were then varnished using a commercially available epoxy anhydride varnish system (Vitalure ™ 120 manufactured by AkzoNobel). After that, the lacquered sheets were locally deformed for the Ericksen hood test.

[0073] To analyze the characteristics of chromium oxide coated with tinplate, several sterilization tests were performed to evaluate wet adhesion on a flat and deformed material. A total of 5 different sterilization media were used during these tests, as shown in Table 2.

[0074]

table 2 Sterilization Test Conditions Type of Sterilization medium Temperature [° C] Time [min] Saline solution 3.6 wt.% NaCl 121 90 Acetic acid 1 wt.% CH ₃ COOH 121 90 Cysteine 3.56 g / l KH ₂ PO ₄ +7.22 g / l
Na ₂ HPO ₄ ⋅ 2H ₂ O + 0.5 g / l C ₃ H ₇ NO ₂ S⋅HCl⋅H ₂ O
(in buffer solution, pH = 7) 121 90 Salt acid 18.7 g / l NaCl + 30 g / l CH ₃ COOH 121 60 Lactic acid 22.5 g / l C ₃ H ₆ O ₃ 121 60

[0075] After sterilization, the level of adhesion of the varnish to the panels was evaluated (using a transverse cut test and measuring the adhesion force by the adhesive tape method (ISO 2409: 1992 (E)), the formation of bubbles (size and number of bubbles) and apparent discoloration. General results are presented. in table 3, including the characteristics of the control material, which is the same tin plate that was passivated using hexavalent chromium, i.e. the so-called passivated tin plate 311. The characteristics are evaluated on a scale from 0 to 5, where 0 s tvetstvuet excellent performance, and 5 -. a very poor performance results averaged over a number of measurements, which leads to estimates, up to one decimal place.

[0076]

Table 3 Lacquer adhesion test results Type of sterilization ETP-311 (control) ETP - Cr-CrOx Flat Saline solution 2 1,5 Acetic acid four 1,5 Cysteine one one Salt acid 5 one Lactic acid 3 2 Dome Saline solution 2 one Acetic acid 3,5 1,5 Cysteine 4,5 0.5 Salt acid four 0.5 Lactic acid 3 2,5

[0077] The inventors have found that the tinplate variant made in accordance with the invention had the same or better characteristics than the standard tinplate passivated using hexavalent chromium (ie, control). It is surprising that no staining of the gray material according to the invention was found, which is difficult to achieve in the case of conventional passivated tinplate and it is especially difficult to achieve with alternative passivation of tinplate without hexavalent chromium.

[0078] Example 3: a roll of black sheet metal (BP) (steel of ordinary grade and tempering) not containing any metal coating was processed on a production line with a linear speed of 20 m / min. The treatment sequence began with alkaline cleaning of the steel by passing a strip for approximately 10 seconds through a solution containing 30 ml / l of industrial cleaner (Percy P3) and 40 g / l of NaOH, which was maintained at 60 ° C. During strip cleaning, an anode current density of 1.3 A / dm ^{2 was} applied. After washing with deionized water, a steel strip was passed through an acid solution for approximately 10 seconds to activate the surface. The acid solution consisted of 50 g / L H ₂ SO ₄ , which was maintained at 25 ° C. After washing with deionized water, the steel strip was passed into a plating bath containing trivalent chromium electrolyte, maintained at 50 ° C. This electrolyte consisted of: 120 g / l of basic chromium sulfate, 250 g / l of potassium chloride, 15 g / l of potassium bromide and 51.2 g / l of potassium formate. The pH of this solution was adjusted to 2.3, measured at 25 ° C, by the addition of sulfuric acid. The plating bath contained a set of platinum titanium anodes. During the processing of the strip, a cathode current density of approximately 17 A / dm ^{2 was} applied for a little more than 1 s for electrodeposition of the chromium-chromium oxide coating in an amount of 60-70 mg Cr / m ² (measured by XRF) on the surface of black metal. All samples thus obtained possessed a metallic luster. In FIG. Figures 1 and 2 show a typical SEM image that depicts the deposition of very finely dispersed grains of metallic chromium-chromium oxide on a steel surface.

[0079] The material thus obtained was passed through a coating line for applying a commercially available PET film 20 microns thick by heat sealing. After pressing the film, the coated strip was heated to temperatures above the melting point of PET, and then sharply cooled in water at room temperature in accordance with the usual processing method for laminating a PET film on a metal. The same procedure was performed in the manufacture of control material using a commercially available ECCS roll.

[0080] These laminated materials were used to make standard DRD cans for canned food (211 × 400). In all cases, the dry adhesion of the PET film to the can wall was excellent. This was confirmed by measuring the forces of the T-shaped peeling of the PET film from the can wall, which turned out to be similar to the values for the PET film deposited both on the material of the invention and on commercial ECCS (~ 7 N / 15 mm).

[0081] Thereafter, the DRD cans were filled with various media, closed, and sterilized. We processed some cans that had a scratch made on the can wall to model and observe the effect of accidental damage to the coating. An overview of the types of sterilization tests performed is presented in Table 4.

Table 4 Sterilization Test Conditions Type of Sterilization medium Temperature [° C] Time [min] Saline solution 3.6 wt.% NaCl 121 60 Acetic acid 1 wt.% CH ₃ COOH 121 60 Cysteine 3.56 g / l KH ₂ PO ₄ +7.22 g / l
Na ₂ HPO ₄ ⋅ 2H ₂ O + 0.5 g / l
C ₃ H ₇ NO ₂ S⋅HCl · H ₂ O (in buffer solution, pH = 7) 130 60

[0082] After the sterilization treatment, the DRD cans were cooled to room temperature, emptied, washed and dried for one day. The bottom and wall of the can were visually evaluated for the presence of spots of corrosion and bloating. The results presented in table 5 indicate that the sterilization characteristics of the material according to the invention are generally slightly lower compared to the ECCS control. This material seems, in particular, more susceptible to corrosion / peeling of the coating after damage to the coating. However, these sterilization tests were quite stringent, so that in practice the material of the invention can be used in specially selected applications, including sterilization.

[0083] The characteristics are rated on a scale from 0 to 5, where 0 corresponds to excellent characteristics and 5 to very poor characteristics.

[0084]

Table 5 Sterilization Test Results Type of sterilization ECCS (control) BP - Cr-CrOx Saline solution eleven)* fourteen)* Acetic acid one 3 Cysteine 0 0 * The symbol in brackets refers to DRD cans with a scratch on the can wall.

[0085] Example 4: A sheet of black sheet metal (BP) (steel of ordinary grade and tempering) without any metal coating was processed on a production line identical to that described in the previous example, for coating Cr-CrOx.

[0086] Then, sheets cut from this roll were varnished by applying a commercially available epoxyphenol varnish system (Vitalure ™ 345 manufactured by AkzoNobel). After that, the lacquered sheets were locally deformed for the Ericksen hood test.

[0087] In order to analyze the characteristics of chromium oxide coated sheet metal, several sterilization tests were performed to evaluate wet adhesion on a flat and deformed material. A total of 5 different sterilization media were used during these tests, as shown in Table 6.

[0088]

Table 6 Sterilization Test Conditions Type of Sterilization medium Temperature [° C] Time [min] Saline solution 3.6 wt.% NaCl 121 60 Acetic acid 1 wt.% CH ₃ COOH 121 60 Cysteine 3.56 g / l KH ₂ PO ₄ +7.22 g / l
Na ₂ HPO ₄ ⋅ 2H ₂ O + 0.5 g / l
C ₃ H ₇ NO ₂ S⋅HCl⋅H ₂ O (in buffer solution, pH = 7) 130 60 Salt acid 18.7 g / l NaCl + 30 g / l CH ₃ COOH 121 60 Lactic acid 22.5 g / l C ₃ H ₆ O ₃ one hundred thirty

[0089] After sterilization, the level of adhesion of the varnish to the panels was evaluated (using a transverse cut test and measuring the adhesion force by the adhesive tape method (ISO 2409: 1992 (E)), bubble formation (bubble size and number) and apparent discoloration. General results are presented. table 7, including the characteristics of the control material, which was used commercially available ECCS. The characteristics were evaluated on a scale from 0 to 5, where 0 corresponds to excellent performance, and 5 - very poor performance.

[0090]

Table 7 Sterilization Test Results Type of sterilization ECCS (control)  BP - Cr-CrOx Flat Saline solution 0 0 Acetic acid 0 0 Cysteine 0 0 Salt acid 0 0 Lactic acid 0 0 Dome Saline solution 0 0 Acetic acid 5 four Cysteine 0 0 Salt acid 0 0 Lactic acid 0 0

[0091] The inventors have found that a Cr-CrOx coated black sheet made in accordance with the invention has the same characteristics as conventional ECCS.

[0092] Brief Description of the Drawings:

[0093] FIG. Figures 1 and 2 show typical SEM images that show the deposition of very finely dispersed grains of metallic chromium oxide of chromium on the surface. FIG. 1 relates to a tinplate substrate, and FIG. 2 relates to a black sheet substrate.

[0094] FIG. Figure 3 shows an overview of various packaging applications. The X axis represents grades of packaging steel, and the Y axis represents a typical thickness range for these applications in which a packaging steel substrate could be used in accordance with the invention.

[0095] FIG. 4 shows the dependence of current on the potential of the anode relative to the SHE, and in FIG. 5 shows a schematic drawing of a gas diffusion anode.

Claims (20)

1. A method of processing black sheet for packaging applications, including electrolytic deposition of a coating of metal chromium chromium oxide on a substrate in one step from an electrolyte containing a mixture of basic chromium sulfate, a chelating agent and increasing salt conductivity, the cation in the chelating agent and increasing salt conductivity is an alkaline cation metal, and the chelating agent contains an anion of formic acid, while to ensure the deposition of metallic chromium exceed the threshold cathode density first current to achieve a given level of pH at the substrate surface which is higher than the pH in the electrolyte volume. 2. The method of claim 1, wherein the electrolyte also comprises a depolarizer. 3. The method of claim 2, wherein the depolarizer comprises a bromide-containing salt. 4. The method of claim 2, wherein the cation in the depolarizer is an alkali metal cation. 5. The method according to p. 1, in which the cation in the chelating agent and increases the conductivity of the salt is potassium. 6. The method according to p. 4, in which the cation in the chelating agent that increases the conductivity of the salt and the depolarizer is potassium. 7. The method according to claim 1, wherein the coated substrate is further provided with an organic coating on one or both sides of the thermosetting organic coating in a varnishing step or a thermoplastic single layer or thermoplastic multilayer polymer coating in a film pressing step or a direct extrusion step, preferably wherein the thermoplastic polymer coating is a polymer coating system comprising one or more layers containing thermoplastics, such as complex poly Fira or polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon polymers, polycarbonates, styrene resins, ABS plastics, chlorinated polyethers, ionomers, urethane resins and functionalized polymers and / or copolymers and / or mixtures thereof. 8. The method of claim 1, wherein an anode is selected that reduces or eliminates the oxidation of Cr (III) ions to Cr (VI) ions in the electrodeposition step, such as a gas diffusion anode. 9. The method according to p. 1, in which the black sheet is subjected to recrystallization annealing black sheet of single or double rolling. 10. The method according to p. 1, in which the black sheet is a cold rolled and subjected to reductive annealing black sheet. 11. The method according to p. 1, in which, by electrolytic deposition, a layer of metallic chromium-chromium oxide is deposited on black metal with a total chromium content of at least 20 mg / m ² , preferably at least 40 mg / m ² , and more preferably at least 60 mg / m ² . 12. The method according to p. 1, in which, by electrolytic deposition, a layer of metallic chromium oxide of chromium is deposited on black metal with a total chromium content of not more than 140 mg / m ² , preferably not more than 110 mg / m ² . 13. The method of claim 1, wherein the electrolyte contains a surfactant, to which an acid or base is optionally added to adjust the pH. 14. The method according to p. 1, in which the electrolyte does not contain a buffer substance. 15. A black sheet substrate for packaging applications, wherein one or both sides of the substrate is coated with a metal chromium-chromium oxide coating layer obtained in one step using a trivalent chromium electrodeposition process, wherein the coated substrate is further provided with an organic coating consisting of a thermoplastic multilayer polymer coating, preferably the thermoplastic polymer coating is a polymer coating system or polymer coatings, including one sludge more layers containing thermoplastics such as polyesters or polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon polymers, polycarbonates, styrene resins, ABS plastics, chlorinated polyethers, ionomers, urethane resins and functionalized polymers, and / or their copolymers, and / or mixtures thereof. 16. The substrate according to p. 15, in which the black sheet is subjected to recrystallization annealing black sheet single or double rolling. 17. The substrate according to p. 15, in which the black sheet is a cold-rolled and re-annealed black sheet. 18. The substrate of claim 15, wherein the metal chromium oxide layer has a total chromium content of at least 20 mg / m ² , preferably at least 40 mg / m ² , and more preferably at least 60 mg / m ² . 19. The substrate according to p. 15 or 18, in which the layer of metallic chromium-chromium oxide has a total chromium content of not more than 140 mg / m ² , preferably not more than 110 mg / m ² . 20. The substrate according to p. 15 or 18, in which the organic coating consists of a thermoplastic single or multilayer polymer coating, and the thermoplastic polymer coating is a polymer coating system or polymer coatings comprising one or more layers containing a complex polyester and / or its copolymers and / or mixtures thereof.

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